The present invention relates to a semiconductor photodetector (hereafter referred to in abbreviation as "PD").
In recent years, side-entry type PDs with light to be detected entering in the horizontal direction relative to the substrate have been proposed as semiconductor photodetectors that are well suited for flat mounting modules. Generally speaking, the main objective of flat mounting modules is reduction of production cost, and implementation of non-alignment (passive alignment) mounting by using inexpensive optical elements.
In passive alignment mounting, precision alignment for perfectly matching the optical axes of the individual optical elements to be mounted is not implemented. As a result, a semiconductor photodetector to be employed in a flat mounting module must demonstrate outstanding tolerance characteristics so that sufficient sensitivity is assured even if misalignment of optical axes occurs within the mounting accuracy range of the passive alignment mounting method.
It is to be noted that tolerance characteristics in this context refer to characteristics representing the optical axis misalignment tolerance range over which a sufficient degree of photodetection sensitivity is achieved, and under normal circumstances, different characteristics manifest depending upon the direction in which the optical axis misalignment is measured. In the following explanation and in the attached drawings, a coordinate system is employed to explain the tolerance characteristics of semiconductor photodetectors. In the coordinate system, the X direction represents the direction of the width of the substrate, the Y direction represents the direction of the thickness of the substrate and the Z direction represents the direction of the depth of the substrate, relative to the direction in which light enters.
Generally speaking, a side-entry type PD is provided with a laminated body adopting a structure achieved by directly or indirectly sandwiching a light absorption layer with complementary type semiconductor layers from a vertical direction, with the laminated body functioning as a photodetection portion. In other words, in a side-entry type PD, light is detected by extracting to the outside an electrical charge generated by the entry of light into the light absorption layer of the laminated body via the complementary semiconductor layers.
Side-entry type PDs in the prior art include, for instance, (A) cleavage photodetection plane PDs and (B) light refraction type mesa photodetection surface PDs.
(A) A cleavage photodetection plane PD is a side-entry type PD that uses a cleavage plane formed at a side surface of the laminated body as a photodetection surface.
(1) Cleavage photodetection plane PDs in the prior art include a semiconductor photodetector 600 adopting a simple pin structure illustrated in FIG. 14. FIG. 14 is a cross section in the Y-Z direction that illustrates the schematic structure of the semiconductor photodetector 600 in the prior art. It is to be noted that the term simple pin structure is used to describe a laminated body with a pin junction in which the light absorption layer is directly sandwiched by complementary semiconductor layers.
As illustrated in FIG. 14, the semiconductor photodetector 600 in the prior art adopts a structure in which a laminated body 620 having a cleavage photodetection plane 620a formed at a side surface thereof is provided on, for instance, an n.sup.+ -InP substrate 610. In addition, the laminated body 620 is formed by sequentially laminating a buffer layer 630, a light absorption layer 640 and a cap layer 650 on the substrate 610.
During the process for manufacturing the semiconductor photodetector 600 in the prior art, the buffer layer 630 which is equivalent to a semiconductor layer on the substrate side is formed by, for instance, epitaxially growing an n-InP layer on the substrate 610. In addition, the light absorption layer 640 is formed by, for instance, epitaxially growing an n.sup.- -InGaAsP layer on the buffer layer 630. The cap layer 650 which is equivalent to a semiconductor layer is formed by, for instance, epitaxially growing a p.sup.+ -InP layer on the light absorption layer 640.
In the semiconductor photodetector 600 in the prior art adopting this structure, incoming light P6 entering through the cleavage photodetection plane 620a is not wave-guided to the inside of the laminated body 620. In other words, with the semiconductor photodetector 600, in which the incoming light P6 is completely absorbed at the light absorption layer 640 in the vicinity of the cleavage photodetection plane 620a, degradation of the cutoff frequency due to a local increase in the electrical charge density tends to occur.
(2) An example of cleavage photodetection plane PDs with cutoff frequency characteristics superior to those of the cleavage photodetection plane PDs having the simple pin structure described above is a side-entry type PD adopting a waveguide structure disclosed by M. Shishikura et al. In Electron. Lett. vol. 32, No. 20, p1882-1883, 1996. It is to be noted that the waveguide structure in this context refers to a structure having a light guide layer for guiding light into the inside of the laminated body between the light absorption layer and the semiconductor layer, i.e., a laminated structure in which the light absorption layer is indirectly sandwiched by semiconductor layers.
Now, a side-entry type PD adopting the waveguide structure in the prior art is explained in reference to a semiconductor photodetector 700 illustrated in FIG. 15. It is to be noted that FIG. 15 is a cross section in the Y-Z direction illustrating a schematic structure of the semiconductor photodetector 700.
As illustrated in FIG. 15, the semiconductor photodetector 700 in the prior art adopts a structure achieved by forming a laminated body 720 having a cleavage photodetection plane 720a formed at a side surface thereof on, for instance, an n.sup.+ -InP substrate 710. The laminated body 720 is formed by sequentially laminating a buffer layer 730, a first light guide layer 735, a light absorption layer 740, a second light guide layer 745 and a cap layer 750 on the substrate 710.
During the process for manufacturing the semiconductor photodetector 700, the buffer layer 730 which is equivalent to a semiconductor layer, is formed by, for instance, epitaxially growing an n-InP layer on the substrate 710. In addition, the first light guide layer 735 is formed by, for instance, epitaxially growing an n-InGaAsP layer on the buffer layer 730. The light absorption layer 740 is formed by, for instance, epitaxially growing an n.sup.- -InGaAsP layer on the first light guide layer 735.
Furthermore, in the semiconductor photodetector 700, the second light guide layer 745 is formed by, for instance, epitaxially growing an n-InGaAsP layer on the light absorption layer 740. The cap layer 750 which is equivalent to a semiconductor layer is formed by, for instance, epitaxially growing a p.sup.+ -InP layer on the second light guide layer 745.
In the semiconductor photodetector 700 structured as described above, an incoming light P7 is wave-guided to the inside of the laminated body 720 by the first light guide layer 735 and the second light guide layer 745 unlike in the semiconductor photodetector 600 adopting the pin structure illustrated in FIG. 14. Therefore, degradation of the cutoff frequency due to a local increase in the electrical charge density does not occur so readily. As a result, the cleavage photodetection plane PD adopting the waveguide structure achieves outstanding characteristics with respect to the cutoff frequency, and is thus commonly employed as a side-entry type PD in the prior art together with the light refraction type mesa photodetection surface PD which is to be explained below.
Now, the tolerance characteristics in the direction of the X axis at a cleavage photodetection plane PD are determined depending exclusively upon the shape of the laminated body constituting the photodetection portion. In addition, the tolerance characteristics in the direction of the Y axis are determined, exclusively depending upon the thicknesses of the individual layers in the laminated body. Furthermore, the tolerance characteristics in the direction of the Z axis are determined, depending upon both the shape of the laminated body and the thicknesses of the individual layers in the laminated body. Thus, in a cleavage photodetection plane PD for application in a flat mounting module, it is necessary to increase the thicknesses of the individual layers constituting the laminated body in order to improve the tolerance characteristics in the direction of the Y axis and the tolerance characteristics in the direction of the Z axis.
Normally, the production costs of a wafer formed by epitaxially growing individual layers increase in proportion to the total number of layers to be epitaxially grown and the total thickness of the laminated structure, resulting in an increase in the prime cost. Consequently, a cleavage photodetection plane PD achieving good tolerance characteristics will be expensive. Particularly, a waveguide type cleavage photodetection plane PD, in which the total number of layers to be epitaxially grown is larger than that in a simple pin structure, is even more expensive.
(B) Next, another side-entry type PD in the prior art, i.e., a light refraction type mesa photodetection surface PD is explained. The light refraction type mesa photodetection surface PD, which is a side-entry type PD having a mesa surface formed to constitute a photodetection surface at a side surface of a substrate, guides light from the substrate to the laminated body by utilizing the refraction of the light at the mesa surface. Namely, in the light refraction type mesa photodetection surface PD, light enters the surface of the light absorption layer toward the substrate, unlike in the cleavage photodetection plane PD described earlier.
As a result, the size of the area over which light enters at the light absorption layer increases so that any local increase in the electrical charge density at the light absorption layer can be prevented without having to employ the waveguide structure for the photodetection portion. Thus, the light refraction type mesa photodetection surface PD simply requires that a photodetection portion adopting the simple pin structure be employed, thereby achieving both a reduction in the initial cost and prevention of degradation in the cutoff frequency.
Furthermore, since the size of the area over which light enters at the light absorption layer is increased, the required photodetection sensitivity is assured even if the position at which light enters the mesa surface is offset to a degree. Consequently, the light refraction type mesa photodetection surface PD normally achieves better tolerance characteristics than the cleavage photodetection plane PD.
Mesa photodetection surface PDs in the prior art include a reverse mesa photodetection surface PD using a reverse mesa surface as its photodetection surface and a forward photodetection surface PD using a forward mesa surface as its photodetection surface.
(1) The reverse mesa surface in this context refers to an inclined surface (mesa surface) formed through etching to face toward the inside of the substrate. The reverse mesa surface may be formed at a side wall in the vicinity of the entrance to a groove with a barrel-shaped cross section formed on a substrate through wet etching implemented by utilizing anisotropy between the direction of the crystal and the direction in which the etching advances.
Reverse mesa photodetection surface PDs in the prior art include, for instance, that disclosed by H. Fukuno et al. in Electron. Lett vol. 32, No. 25, p2346-2348, 1996.
At a reverse mesa photodetection surface PD, the size of the reverse mesa surface must be large in order to improve the tolerance characteristics with respect to misalignment. Normally, a diffusion rate controlled etching solution is used on an InP substrate which is employed as a substrate for optical elements. Since, in the barrel-shaped groove where the reverse mesa surface is formed (the reverse mesa portion), the etching solution that has entered the groove is not readily diffused, etching does not ultimately progress in the direction of the depth of the groove and only the entrance of the groove expands even if the object is to deepen the groove. Thus, it is extremely difficult to grow a reverse mesa surface over a large area. Under normal circumstances, irregular increases and decreases in the size of the reverse mesa surface result.
In addition, when the depth of the barrel-shaped groove is increased to form a large reverse mesa surface to improve the tolerance characteristics, it becomes very difficult to remove the wax which enters the groove during a polishing step performed at the substrate rear surface in order to achieve a specific chip thickness. Consequently, the wax often remains adhered to the reverse mesa surface which constitutes the photodetection surface in a reverse mesa photodetection surface PD. This adhesion which often results in degradation in photodetection efficiency.
(2) With another type of light refraction type mesa photodetection surface PD, i. e. , a beveled entry type PD (forward mesa photodetection surface PD), the photodetection surface can be machined and formed with a greater degree of ease compared to the reverse mesa photodetection surface PD described above. It is to be noted that the forward mesa surface in this context refers to a mesa surface formed toward the outside of the substrate as opposed to the case with the reverse mesa surface. Beveled entry type PDs in the prior art include, for instance, the photodetector disclosed by Norimatsu et al. in the 1996 Shingakukai Society Convention, C-218. This beveled entry type PD in the prior art is now explained in reference to FIG. 16 which illustrates a semiconductor photodetector 800.
It is to be noted that FIG. 16, which is referred to in the explanation of the semiconductor photodetector 800 is a cross section in the Y-Z direction illustrating a schematic structure of the semiconductor photodetector 800. In addition, FIGS. 17-19 present cross sections in the Y-Z direction illustrating schematic structures of a semiconductor photodetector 900 achieved by reducing the size of the photodetection portion in the semiconductor photodetector 800 to reduce the junction capacity.
As illustrated in FIG. 16, a laminated body 820 adopting a pin structure, which is equivalent of a photodetection portion, is formed on a substrate 810 with a forward mesa surface 810a formed at a side surface thereof. At the semiconductor photodetector 800, incoming light P8 entering the forward mesa surface 810a is refracted at the forward mesa surface 810a and is then transmitted diagonally through the substrate 810. Then, the incoming light P8 enters a light absorption layer 840 via a buffer layer 830 to be absorbed at the light absorption layer 840.
In the semiconductor photodetector 800 in the prior art, the angle of incidence of light entering the laminated body 820 is dependent upon the inclination of the forward mesa surface 810a relative to the rear surface of the substrate 810, which is determined by the etching solution used and the refraction factor of the substrate 810 determined by the material constituting the substrate 810. As indicated in the publication mentioned above, in which the light which has been refracted at the forward mesa surface advances at approximately 25.12 degrees relative to the direction of entry, the angle of incidence of light into the laminated body 820 is normally small.
As a result, if the semiconductor photodetector 800 is formed as a semiconductor photodetector 900 illustrated in FIG. 17 in order to improve its cutoff frequency and if a polishing error occurs while polishing the rear surface of the substrate 820, incoming light P9 will not enter the laminated body 920 constituting the photodetection portion, as illustrated in FIG. 18 or 19.
As has been explained, it is obvious that it is difficult to achieve a further improvement in the cutoff frequency with the beveled entry type PD since its photodetection portion must be formed large due to the consideration of possible polishing error at the substrate rear surface.